WEDNESDAY, Jan. 29, 2014 (HealthDay News) -- Comparisons between modern humans and Neanderthals are usually meant as either an insult or a joke. But a new study suggests that many people today still harbor bits of Neanderthal DNA that affect their health.

The investigators also found these Neanderthal DNA fragments are found at high levels in genes that determine skin and hair traits, and at low levels in regions of the X chromosome and genes associated with the testes.

The findings show the ways that genetic material inherited from Neanderthals can be both beneficial and harmful for modern humans, according to the authors of the study, which was published Jan. 29 in the journal Nature.

"Now that we can estimate the probability that a particular genetic variant arose from Neanderthals, we can begin to understand how that inherited DNA affects us," study senior author David Reich, a professor of genetics at Harvard Medical School, said in a Harvard news release.

"We may also learn more about what Neanderthals themselves were like," Reich said.

Previous research has shown that Neanderthals interbred with early modern humans in Europe and Asia 40,000 to 80,000 years ago, and that about 2 percent of the genetic material of modern people without African ancestry can be traced to Neanderthals. Modern Africans have little or no Neanderthal DNA because their ancestors did not mingle with Neanderthals.

In this study, the researchers examined genetic variants in about 850 people with non-African heritage, nearly 200 people from sub-Saharan Africa, and the 50,000-year-old remains of a Neanderthal.

The investigators discovered that the genetic makeup of modern people with non-African heritage had some regions with high levels of Neanderthal DNA and other areas with far lower amounts of Neanderthal DNA.

Nine genetic variants associated with diseases related to immune function (such as diabetes) and to certain behaviors (such as the ability to quit smoking) likely came from Neanderthals, the researchers said. They added that more variants with Neanderthal origins are likely to be found.

The investigators also found high levels of Neanderthal DNA in genes that affect certain proteins that help make skin, hair and nails tough, and can be helpful in colder regions by providing thicker insulation.

"It's tempting to think that Neanderthals were already adapted to the non-African environment and provided this genetic benefit to humans," Reich said.

Irene Eckstrand, of the U.S. National Institute of General Medical Sciences, added: "The story of early human evolution is captivating in itself, yet it also has far-reaching implications for understanding the organization of the modern human genome."

The U.S. National Institute of General Medical Sciences partially funded the study.

"Every piece of this story that we uncover tells us more about our ancestors' genetic contributions to modern human health and disease," Eckstrand said in the news release.